1. Field of the Invention
This invention relates generally to ceramic processing and more specifically it relates to ceramic processing using dispersants that improve the rheological properties of ceramic slurries.
2. Description of the Prior Art
Much of the commercial ceramic, including ferrite production, uses a processing sequence that compacts dry granular ceramic powder. Although several techniques may be used to obtain appropriately granulated powder, the principal commercial technique used today spray dries a ceramic slurry to produce generally spherical agglomerated grains of uniform and controllable size which lead to the better flow and die filling properties necessary for ceramics of uniform density and microstructure.
The slurry that is spray dried is usually prepared by ball milling a solution, typically aqueous, containing the ceramic materials. To increase the dispersion of the solids and thus maximize the solids concentration within the slurry, a dispersant is normally added to the slurry to decrease the slurry viscosity without the necessity of adding excessive water. The addition of excessive water to the slurry to lower the viscosity is not desirable for several reasons. Spray drying is an energy intensive process and production costs necessarily increase. Excessive water also produces lower density granules and higher losses because of the presence of fine agglomerates. The dispersant also improves packing when the ceramic is pressed. Some ceramics are processed without spray drying but a dispersant is added to the slurry to facilitate ball milling or other subsequent processing.
A good dispersant must satisfy several criteria. It should minimize the amount of water required to obtain the desired viscosity and be compatible with other additives and processing steps. Since an organic binder is often used to produce the strength and plasticizing properties needed for dry processing before sintering, the dispersant must be compatible with the binder. The dispersant is not a desirable addition to the final ceramic composition and it should be easily removable at some point in the processing sequence.
A commonly used dispersant is gum arabic which is a natural product collected from trees belonging to the genus acacia. Species of this genus have an extensive geographical range but trees growing in the Sudan and Senegal generally produce the best gum arabic. Gum arabic has generally good dispersing properties but unfortunately also has several undesirable properties. In addition to possibly uncertain supply, its properties are not easily reproducible. Gum arabic contains relatively large quantities of inorganic materials, such as silicon, sodium and calcium, some of which can adversely alter the properties of the ceramic composition.